A conventional "abnormal grain growth annealing" heat treatment has been used to produce low (0.009 wt %) and high (0.12 wt %) nitrogen single crystal line Fe-40Ni-15Cr wt % base alloys. Constant strain-rate tensile tests were carried out in the temperature range between 77 and 298 K at two different strain-rates. The results obtained were analysed using the standard procedure for the thermally activated glide of dislocations, and the possible rate-controlling mechanisms for athermal and thermal nitrogen-induced strengthening have been discussed. A conventional two-beam bright-field transmission electron microscope was next used to determine the character of the dislocations and their dependence on the amount of nitrogen in the alloy. It was found that the dislocations became predominantly screw in character as the nitrogen content in the alloy was increased. These findings have been discussed in the light of the existing models for the nitrogen strengthening of the Fe-Ni-Cr austenite.